This invention relates to a method for operating a mass flowmeter that employs the Coriolis principle, said mass flowmeter incorporating a measuring tube through which flows a fluid medium and which is stimulated to oscillate in a predefined pattern, allowing the resulting oscillatory response of the measuring tube to be detected and measured.
Coriolis mass flowmeters operated by the method referred to above have been well known in prior art, as described, for instance, in DE 100 02 635 A1. Conventional Coriolis mass flowmeters often employ natural self-resonance, i.e. the measuring tube is energized at a self-resonant frequency, hereinafter also referred to as the natural frequency or mode, with a predefined amplitude.
In single-phase flow-through operation, conventional Coriolis mass flowmeters are highly accurate and very dependable. However, multiphase flow patterns of the medium traveling through the measuring tube can lead to a significant decline of the accuracy and dependability of the Coriolis mass flowmeter. In general, a multiphase flow pattern is constituted of two or more phases each of which features different physical properties. In any such case, the phases may consist of identical or of different substances. The term phases relates to homogeneous, spatially delimited segments of the flowing medium. Examples of a multiphase flow include fluid-and-solids combinations, gas-and-liquid combinations, gas-and-solids combinations, water-and-vapor combinations as well as water-and-air combinations.
As indicated above, multiphase flow measurements can be susceptible to significant errors. The primary cause is the presence of secondary flows in the measuring tube which are essentially attributable to mutually diverging densities of the different phases in the multiphase flow.